Empty channels publish handling

[rtibbles]

Summary

Description of the change(s) you made

• Adds preemptive handling of channels that are completely empty, to prevent Integrity Errors caused by trying to create a ChannelMetadata object when there is no root node to make a Foreign Key to
• Adds a regression test to handle this
• Adds some better clean up of database connections that are created by the content database context manager - otherwise tests that test errors in the publishing process always produce errors during teardown
• Migrates to a custom exception class for errors raised when the root node is not publishable, so it can be specially handled to not set the channel as publishable again

Manual verification steps performed

1. Created a regression test that replicated the issue
2. Fixed the issue
3. Reran the test
4. Profit!

Reviewer guidance

How can a reviewer test these changes?

This doesn't do anything about how someone was able to publish a channel while in this state, I think there's still more to work on there, but we should at least handle these things a bit better until we can fix that up better.

References

Fixes #4214

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Contributor's Checklist

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Reviewer's Checklist

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• Automated test coverage is satisfactory
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